Hereinbelow, as an example of the related art of the present invention, a welding technique for producing welded products using high energy density beams such as laser beams will be described. The high energy density beams have high energy input per unit area of beam, thus effectively increasing the weld strength of weld metal zones in welded products. In recent years, as shown in FIG. 16 and FIG. 17, a laser beam is preferably used for welding a plurality of pieces of sheet material 1X and 2X together to produce a welded product.
Japanese Patent Publication No. Hei. 2-43590 (first cited patent document) discloses a butt seam welding technique, in which ends of thin steel sheets are butted and welded together using a laser beam. In the butt seam welding technique, welding is executed under the condition that the laser beam is transmitted through an optical fiber, with the spot diameter of the beam set to two or more times the thickness of each of the steel sheets, or ten or more times the gap between the butted ends of the steel sheets, and the energy density of the beam set lower than a level capable of generating plasma.
In addition, Japanese Patent Laid-open Publication No. Sho. 62-101382 (second cited patent document) discloses a method of welding the flanges of a pulley to each other, in which an electrode is placed to come into contact with the outer circumferential surfaces of the flanges (the outer diameter end surfaces of the flanges) from the outside, ring-shaped compression discs, which are made of a nonconductive and heat-resistant ceramic material, are placed outside the side surfaces of the respective flanges and compress the flanges in the thickness direction of the flanges, and, thereafter, a welding current flows to the electrode, thus generating Joule heat to weld the flanges together.
According to the above-mentioned related art, as shown in FIG. 17, a weld metal zone 5X is formed between the butted surfaces of the pieces of sheet material 1X and 2X. Further, it is necessary to increase the weld strength and the weld reliability of the weld metal zone 5X. However, when the heat input by a laser beam is increased in order to increase the depth of weld penetration in the weld metal zone 5X, humping is generated in the weld metal zone 5X, as shown in FIG. 18, and thus a humping bead may be formed in the weld metal zone 5X. The technical term “humping bead” means a bead that may be formed in a weld metal zone when molten weld metal in the weld metal zone is humped, thus causing uneven thickness of the weld metal zone in a welded product.
Described in brief, the first and second cited patent documents are problematic in that an increase in the heat input by the laser beam undesirably forms a humping bead in the weld metal zone.